Of the two forms of duplex commonly used in wireless networks, namely frequency division duplex (FDD) and time division duplex (TDD), current Long-Term Evolution (LTE) standards accommodate both FDD (LTE FDD) and TDD (LTE TDD) implementations in half duplex mode. Interference, caused by both a user equipment (UE) itself, as well as inter-terminal interference, present obstacles to implementations of full-duplex communication.
Currently, for all transmission between UEs and base stations, the timeline for a downlink and an uplink transmission is partitioned into units of radio frames. Each radio frame is partitioned into a predetermined number of subframes. For TDD, each subframe used for the downlink may be referred to as a downlink (DL) subframe, and each subframe used for the uplink may be referred to as an uplink (UL) subframe.
A particular TDD DL/UL subframe configuration is associated with a network, a cell or a cluster of cells. In current implementations, such as LTE, the subframe configuration is shared amongst a large number of cells over a larger geographical region. Further, subframe configurations are selected from a limited set of DL/UL subframe permutations. The network usually determines the subframe configuration based on an average need of all UEs in that region. Once chosen, the particular frame configuration is used to serve all UEs belonging to the region, sometimes termed fixed TDD. Thus, the network has centralized control over the frame configuration applied to all UEs served by the network. In other words to avoid base station (BS) to base station (BS-BS) interference and UE-UE inter-cell interference, all TDD deployments typically operate synchronously.
Revisions to the LTE standard include the possibility of dynamically adapting TDD DL/UL subframe configurations based on the actual traffic needs, termed flexible TDD. The revisions allow mutually different subframe configurations across neighboring cells or neighboring clusters of cells. For example, during a short duration, a large data burst on the downlink may be needed; all the UEs served by a particular cell may be instructed by the serving node to change their configuration from one of the known configurations to another predetermined configuration. While flexible TDD may provide more efficient use of resources resulting in UL and DL throughput gain, it may cause interference to both DL and UL transmission when the cells have different overlapping DL and UL subframes, usually at the cell boundary. This inter-layer (DL2UL and UL2DL) interference may significantly affect the overall system performance. Various interference mitigation (IM) techniques may be implemented, such as scheduling dependent IM (SDIM) and cell clustering IM (CCIM). Traffic adaptation (TA) techniques may also be used, that is, proper choice of subframe configuration.
While Flexible TDD is an improvement over fixed TDD in terms of network spectral efficiency and efficient radio resource usage, both techniques are still inefficient in that individual UEs may have needs for DL and UL subframes in proportions that do not correspond to the configuration of the cell. Further, in practical non-full buffer/bursty traffic, some subframes or other network resources may not be fully utilized. Also, for most networks, once a subframe configuration is set, it is generally fixed among geographically adjacent cells over a longer duration even though, as described above, this may compromise spectral efficiency and efficient radio resource usage. Further as described above, interference, caused by both a UE, as well as inter-terminal interference, present obstacles to implementations of full-duplex communication.